Amplifier



K. R. WENDT AMPLIFIER Nov. 4, 1941.

Filed Dec. 30, 19,38 2 Sheets-Sheet 1 Zhmentor (Ittorneg Nov. 4, 1941.

K. R. WENDT, 2,261,787

AMPLIFIER Filed Dec. 50, l938 2 Sheets-Sheet? W 3nnentor (Ittorneg Patented Nov. 4, 1941 AMPLIFIER Karl R. Wendt, Audubon, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application December 30, 1938, Serial No. 248,576

3 Claims.

My invention relates to amplifiers and particularly to wide band amplifiers such as those employed in television or the like. I

As a specific example, my invention may be applied to a television video amplifier where the cathode ray receiver tube is to be operated with the second anode at ground potential. It is desirable to operate a cathode ray tube in this way when the greater part of the tube envelope is of metal as in the case of a so-called demountable tube.

In amplifiers of the above-mentioned type it may be difilcult to obtain the desired fiat response characteristic by following conventional practice, especially if high voltages are involved. Conventional practice requires that the plate resistor or other plate impedance unit have a comparatively low impedance and that the coupling condenser be of large capacity if the frequency response curve is to be fiat over a wide range. It also requires that if the cathode ray tube is supplied with voltage from a filter-rectifier unit or S. P. U. (socket power unit) the filter condenser or condensers be of large capacity, High voltage condensers having the required large capacity are expensive, diflicult to obtain and undesirably large in size.

It is accordingly, an object of my invention to provide an improved amplifier of the above-mentioned type.

A further object of my invention is to provide a wide band amplifier in which comparatively small coupling and filter condensers may be utilized.

It is a further object of my invention to provide an improved amplifier having a very low frequency response.

In a preferred embodiment of my invention there is employed an amplifier tube having high cathode-plate impedance and provided with an external plate circuit of such high impedance that it does not load the tube any substantial amount. The succeeding tube is provided with a grid resistor of such low impedance that it loads the preceding tube and the two tubes are coupled through a comparatively low capacity coupling condenser.

As will be explained more fully hereinafter, in a system where the cathodes of the tubes are maintained at a high difference of potential, this design results in an amplifier having the desired frequency response and, at the same time, the S. P. U. does not feed any substantial hum voltage into the amplifier even though the filter condenser is a small high voltage condenser.

The invention will be better understood from the following description taken in connection with the accompanying drawings in which Figure 1 is a circuit diagram of one preferred embodiment of my invention,

Figure 2 illustrates a modification of the circuit shown in Fig. 1,

Figure 3 is a circuit diagram of another embodiment of my invention, and

Figure 4 illustrates another embodiment of my invention.

Referring to Fig. 1, the invention is shown applied to a television receiver in which there is employed a. cathode ray tube I having a metal envelope which is grounded. The tube 1 includes the usual indirectly heated cathode 2, a control electrode 3, a first anode 4, and a second anode 5.

The wide band picture signal amplifier for supplying signal to the tube l comprises an amplifier tube 1 having a high plate impedance such as a screen grid tube of the 6J7 type illustrated. The succeeding amplifier tubes 8 and 9 may conveniently be screen grid tubes of types SJ? and 6L6, respectively.

In the circuit shown, a negative voltage of 10,000 volts is to be applied to the cathode 2 of the cathode ray tube. This voltage is supplied from a, high voltage rectifier unit (not shown) and a filter unit I I, the two units forming a conventional S. P. U. The filter unit ll comprises filter condensers l2 and C1 and a choke coil IS.

The negative terminal of the S. P. U. is connected through a conductor I4 to the cathode 2 of the cathode ray tube while the positive terminal is grounded. Thus a high voltage is maintained between the cathode 2 and the second anode 5 which is connected to the grounded metal envelope.

A suitable plate voltage is applied from a voltage source such as a battery IE to the plate of the amplifier tube 1 through a high impedance external plate circuit which may include a choke coil L and a plate resistor R1. The impedance of choke coil L and resistor R1 in series at the lowest frequency to be passed by the amplifier should be so great as not to load the tube 1 any substantial amount.

The coupling between tubes 1 and 8 is through a high voltage condenser C of comparatively small capacity. The tube 8 is provided with grid leak resistor R having such low resistance that the resistor R acts as the load on the tube 1. Using the high impedance plate circuit L, R1 and high impedance tube 1, the capacity C and resistance R of the units C and R, respectively, may

be given such values that if the same values were used in a conventional amplifier, only one or two percent of the desired low frequency signal would be passed, whereas the same values in the circuit illustrated give a fiat response.

Also, the filter condenser C1 may be of comparatively small capacity since most of the voltage variation across this condenser appears across the high impedance units L, R1 and C rather than across the low impedance unit R. The voltage variation across condenser C1 includes 120 cycle hum or ripple, line voltage variations and some video signal. It is, of course, the portion of this voltage variation which appears across R which is amplified to cause amplifier difficulties. For example, if too much video signal is fed back through C1 to the resistor R, the amplifier may oscillate.

From the foregoing, it will be evident that the condensers C and C1 which must be designed to withstand a high voltage may have a fairly low capacity.

The above-described amplifier may have circuit constants as follows, these being given merely by way of example:

L =10,000 henrys R1=5,600 ohms R =2,200 ohms C1=0.1 microfarad C,=L034 microfarad The coupling between the amplifier tubes 8 and 9 and between the tubes 9 and I may be of conventional design. In the particular circuit shown there is provided direct current coupling between tubes 9 and I, a potentiometer [8 being provided across a voltage source l9 for the purpose of adjusting the bias voltage on the control electrode 3. The usual by-pass condenser 2| preferably is provided which may be a low volt age 4 microfarad condenser.

It may be noted that, in general, the impedance of the choke coil L should be about thirty times that of condensersv C and C1 in series at the lowest frequency to be passed by the amplifier without phase shift.

In Fig. 2 there is shown a circuit which is like that shown in Fig. 1 except that a high. im-i pedance resistor III has been substituted for the impedance units R1 and L. Like parts in. the

two figures have the same reference numerals. It will be understood that in Fig. 2 the resistor ID has an impedance comparable to that of R1 and L in series and that it is high compared with resistor R.

In Fig. 3 there is shown another embodiment of my invention. It is common practice in designing television amplifiers to supply plate voltage to at least one of the amplifier tubes through a filter resistor shunted by a filter condenser. In such a circuit the low frequency response is held up due to the impedance of the filter condenser increasing with decrease in frequency.

By employing the circuit of Fig. 3, an amplifier may be made to have a much lower frequency response than is possible as a, practical matter where the above-mentioned common practice is employed.

Referring to Fig. 3, there is shown anamplifier designed for use in either a television receiver or cathode ray tube oscillograph, for example. The amplifier comprises a high impedance amplifier tube 26 which has its plate voltage supplied through a high impedance resistor R from a, voltage sourcesuch as an S. P. U. 27.

The amplifier tube 26 is coupled to amplifier tube 23 through a coupling condenser C. As in the previously described circuits, the grid circuit of the tube 28 is of such low impedance at the lower frequencies to be passed by the amplifier that it provides substantially the only load across the tube 26. This grid circuit includes a grid resistor R'1 which is connected in series with a comparatively high impedance filter resistor R'z and a suitable biasing source such as a battery 29. The filter resistor R'z is shunted by a filter condenser C'1. If desired, a peaking coil 3| may be included in the grid circuit.

Assuming that the reference letters for the several units also represent the values of these units in microfarads and ohms, the amplifier should be so designed that RC'=R1C1. Then the low frequency cut-off of the amplifier equals Rfzci. Since the condenser C1 may be a low voltage condenser, it may easily be designed to have a very large capacity, its'capacity in the,

specific amplifier illustrated being 14 microfarads.

The several pertinent values for the amplifier of Fig. 3 have been indicated on the drawings, by way of example. This amplifier, with the values given, is flat down to 60 cycles, and it will be seen that both the coupling condenser C and the filter resistor Rz have comparatively low values.

Fig. 4 illustrates a variation of the circuit shown in Fig. 3, like parts in the two figures having the same reference characters. In Fig. 4 the tube 28 is provided with a self-biasing resistor 34 and the filter condenser C1 is connected directly to the cathode of tube 28 whereby it acts as a by-pass condenser for resistor 34. The values of the various circuit elements are the same as in Fig. 3, the grid circuit of tube 28 being of low impedance as compared with the plate circuit of the preceding tube.

It will be apparent from the foregoing that various modifications may be made in my invention without departing from. the spirit and scope thereof.

I claim as my invention:

1. An amplifier comprising a high impedance amplifier tube having a plate and a cathode, a second amplifier tube having a control grid and a cathode, a coupling condenser connected between said plate and said grid, means for applying an operating voltage to said plate through a high impedance plate circuit, a low impedance grid resistor connected between the control grid and cathode of said second tube, an electric discharge device having a cathode and having an anode which is connected to ground, and a high voltage source for said device having its positive terminal connected to ground and to the cathode of said first tube and having its negative terminal connected to the cathode of said second tube and to the cathode of said electric discharge device.

2. A television receiver or the like comprising a cathode ray tube having a control electrode and having a cathode which is to be operated below ground potential and an anode' which is connected to ground, a first amplifier tube of the high impedance type having a plate and having a cathode which is connected-to ground, a second amplifier tube having a control grid and a cathode, a coupling condenser connected between said plate and said grid, a source of operating potential for said plate connected between ground and said plate through a circuit having high impedance to signals to be amplified, a low impedance resistor connected between the grid and cathode of said second amplifier tube, a high voltage source for said cathode ray tube having its positive terminal grounded and having its negative terminal connected to the cathode of said second amplifier tube and to the cathode of said cathode ray tube, and means for supplying the signal output of said second amplifier tube to the control electrode of said cathode ray tube.

3. An amplifier comprising a first amplifier 1O tube having a plate and. being of the high impedance type, a second amplifier tube having a control electrode and a cathode, a coupling condenser which couples said plate to said control electrode, means for supplying an operating volt- 15 age to said plate through a comparatively high impedance circuit, a grid resistor of comparatively low impedance connected between said control electrode and ground in series with a comparatively high impedance filter resistor, a selfbiasing resistor connected in the cathode circuit of said second tube between the cathode and ground, and a filter condenser connected between the cathode of said second tube and the junction point of said grid resistor and said high impedance filter resistor whereby said last condenser functions as a by-pass for said self-biasing resistor and also to increase the low frequency response of said amplifier.

KARL R. WENDT. 

